1. Field of the Invention
This invention pertains generally to the field of cockpit indicators or display units that provide flight information to the pilot or flight crew of an aircraft, and more particularly to highway in the sky (“HITS”) pathway depictions on one or more display units providing an artificial horizon through a synthetic vision system (“SVS”), an enhanced vision system (“EVS”), a combined SVS-EVS vision system, and/or a vision system that may produce a classic electronic primary flight display (“PFD”).
2. Description of the Related Art
Cockpit indicators may provide navigation and/or course information to a pilot which allows the pilot to determine if the aircraft is flying on course with a flight plan. Traditional or classic indicators have been driven using signals generated using ground-based navigation aids that are received by the aircraft. Data represented in the signals is used to provide course information to the pilot by depicting or providing vertical and/or horizontal bars (i.e., “needles”) where the pilot is required to “fly the needles” to stay on course. If the aircraft was on course, the needles would be centered on the indicator.
With the existence of a satellite-based navigation system such as global positioning system (“GPS”) and with the growing ability to depict the scene outside of the aircraft with a synthetic and/or enhanced image on a tactical display unit such as a primary flight display (“PFD”), there has been a trend in the aviation community to replace needles with a HITS pathway for providing course information. In one form, a HITS pathway comprises a “tunnel” where the pilot is required to “fly the tunnel” to stay on course.
The depiction of the tunnel is based on, in part, the flight plan between successive navigation fixes (e.g., waypoints) that may or may not have corresponding altitude constraints. A system such as a flight management system (“EMS”) may receive the flight plan and generate flight plan data divided into lateral flight plan and vertical flight plan components. When this information is made available to a flight director system, such system may generate guidance information data and provide such data to a display system which, in turn, presents course guidance information in the form of flight director symbology so that the aircraft can fly an FMS-generated flight plan if the flight director is set to an “auto” mode. Often, the flight director system is part of an autoflight system that also provides guidance signals to an autopilot system which, in turn, causes the aircraft to fly the flight plan automatically.
Although an autoflight system is beneficial to the pilot, there are times when the pilot may want to fly a flight plan that is different from an FMS-generated flight plan and the HITS pathway. For instance, an FMS may determine a vertical flight plan for a departure course between the take-off and cruise phases of flight, but the pilot may wish to climb to a higher altitude at a faster rate than the rate determined by an FMS. At such a time, a pilot may switch to a “manual” mode of the flight director, and based on manually-selected inputs directed to the flight director, a flight director may provide guidance information data reflecting a higher path angle corresponding to a higher rate of climb; however, the HITS tunnel would remain at its current position, causing a divergence between the flight director and the HITS tunnel. For a period of time, the pilot may intentionally steer the aircraft to a higher path attitude as directed by the flight director and away from the tunnel; that is, for a HITS pathway depicted by a tunnel, flight director guidance could cause the pilot to not “fly the tunnel.”
The pathway depicted as a HITS may be generated using an Earth-based reference system. Earth-based reference systems may be employed in cruise, arrival, and approach phases of flight with little difficulty because paths associated with these phases of flight may provide a fixed flight level or glide path angle in the vertical flight plan. While these phases of flight may provide definite guidance for a vertical flight plan, such definite guidance (such as a fixed climb gradient) of an Earth-based reference system is generally not applied in the take-off and/or departure phases of flight. Navigation systems such as an FMS may not create an Earth-based referenced climb gradient; instead, vertical flight plans may be defined using altitude constraints at which an aircraft may cross at, above, or below the navigation fix associated with the constraint. Even if an Earth-based reference is used for a climb gradient, it would be a difficult task for a pilot to fly such a path given other considerations that a pilot must consider on a departure such as maximum speed constraints associated with an airspace surrounding an airport. Moreover, in the interest of fuel economy, it may be beneficial to climb as high as possible and as early as possible using maximum climb thrust. An HITS pathway based on an Earth-based reference system may limit the pilot's ability to “fly the tunnel” while attempting to achieve a greater climb gradient; however, a HITS pathway based on an air mass-referenced system that is provided inherently through various vertical modes of a flight director system may provide the pilot such ability.